On Wed, Sep 02, 2020 at 07:21:52PM -0400, Arvind Sankar wrote: > The CRn accessor functions use __force_order as a dummy operand to > prevent the compiler from reordering CRn reads/writes with respect to > each other. > > The fact that the asm is volatile should be enough to prevent this: > volatile asm statements should be executed in program order. However GCC > 4.9.x and 5.x have a bug that might result in reordering. This was fixed > in 8.1, 7.3 and 6.5. Versions prior to these, including 5.x and 4.9.x, > may reorder volatile asm statements with respect to each other. > > There are some issues with __force_order as implemented: > - It is used only as an input operand for the write functions, and hence > doesn't do anything additional to prevent reordering writes. > - It allows memory accesses to be cached/reordered across write > functions, but CRn writes affect the semantics of memory accesses, so > this could be dangerous. > - __force_order is not actually defined in the kernel proper, but the > LLVM toolchain can in some cases require a definition: LLVM (as well > as GCC 4.9) requires it for PIE code, which is why the compressed > kernel has a definition, but also the clang integrated assembler may > consider the address of __force_order to be significant, resulting in > a reference that requires a definition. > > Fix this by: > - Using a memory clobber for the write functions to additionally prevent > caching/reordering memory accesses across CRn writes. > - Using a dummy input operand with an arbitrary constant address for the > read functions, instead of a global variable. This will prevent reads > from being reordered across writes, while allowing memory loads to be > cached/reordered across CRn reads, which should be safe. > > Tested-by: Nathan Chancellor <natechancel...@gmail.com> > Tested-by: Sedat Dilek <sedat.di...@gmail.com> > Signed-off-by: Arvind Sankar <nived...@alum.mit.edu>
Seems reasonable to me. As reasonable as compiler bug workarounds go, that is. ;) Reviewed-by: Kees Cook <keesc...@chromium.org> -- Kees Cook
